Elsevier

Geochemistry

Volume 66, Issue 1, 31 January 2006, Pages 37-56
Geochemistry

Permo-Carboniferous volcanism in late Variscan continental basins of the Bohemian Massif (Czech Republic): geochemical characteristic

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Abstract

Extensive Permo-Carboniferous volcanism has been documented from the Bohemian Massif. The late Carboniferous volcanic episode started at the Duckmantian–Bolsovian boundary and continued intermittently until Westphalian D to Stephanian B producing mainly felsic and more rarely mafic volcanics in the Central Bohemian and the Sudetic basins. During the early Permian volcanic episode, after the intra-Stephanian hiatus, additional large volumes of felsic and mafic volcanics were extruded in the Sudetic basins. The volcanics of both episodes range from entirely subalkaline (calc-alkaline to tholeiitic) of convergent plate margin-like type to transitional and alkaline of within-plate character. A possible common magma could not be identified among the Carboniferous and Permian primitive magmas, but a common geochemical signature (enrichment in Th, U, REE and depletion in Nb, Sr, P, Ti) in the volcanic series of both episodes was recognized. On the other hand, volcanics of both episodes differ in intensities of Nb, Sr and P depletion and also, in part, in their isotope signatures. High 87Sr/86Sr (0.707–0.710) and low εNd (−6.0 to −6.1) are characteristic of the Carboniferous mafic volcanics, whereas low 87Sr/86Sr (0.705–0.708) and higher εNd ranging from −2.7 to −3.4 are typical of the Permian volcanics. Felsic volcanics of both episodes vary substantially in 87Sr/86Sr (0.705–0.762) and εNd (−0.9 to −5.1). Different depths of magma source or heterogeneity of the Carboniferous and Permian mantle can be inferred from variation in some characteristic elements of the geochemical signature for volcanics in some basins. The Sr–Nd isotopic data with negative εNd values confirm a significant crustal component in the volcanic rocks that may have been inherited from the upper mantle source and/or from assimilation of older crust during magmatic underplating and ascending of primary basic magma. Two different types of primary magma development and formation of a bimodal volcanic series have been recognized: (i) creation of a unique magma by assimilation fractional crystallization processes within shallow-level reservoirs (type Intra-Sudetic Basin) and (ii) generation and mixing of independent mafic and felsic magmas, the latter by partial melting of upper crustal material in a high-level chamber (type Krkonoše Piedmont Basin). A similar origin for the Permo-Carboniferous volcanics of the Bohemian Massif is obvious, however, their geochemical peculiarities in individual basins indicate evolution in separate crustal magma chambers.

Introduction

Late Paleozoic basins of the Bohemian Massif occur in the Rhenohercynian, Saxothuringian and Moldanubian orogenic zones (Fig. 1). The lower part of the sedimentary sequence was deposited in marine or paralic environments (Lower Carboniferous), whereas its upper part consists of continental sediments (Upper Carboniferous and Lower Permian). Besides surface outcrops, the sediments with intercalated volcanics have been documented by thousands of boreholes and provide good constraints on the Westphalian and Stephanian stratigraphy. In addition, Carboniferous rocks are also known from underground mines.

The Bohemian Massif has a unique position within central Europe as the largest exposure of the Variscan Orogen. During the last years, several attempts have been made to characterize the late- to post-collisional volcanic activity and its role in the late stages of the Variscan Orogeny (W. Franke, 1989; D. Franke, 1995). The Variscan Orogeny, culminating at about 325 Ma, was accompanied by the emplacement of a range of subduction- and extension-related magmas (360 and 260 Ma  Lorenz and Nicholls, 1984; Downes and Duthou, 1988; Wilson and Downes, 1991). The mantle beneath the western and central Europe was metasomatized as a consequence of prior plate subduction during the Variscan Orogeny and especially during the late phases of the Late Paleozoic extension (Wilson and Downes, 1991; Wilson, 1993).

In the late Carboniferous, the Bohemian Massif became dissected by a conjugate system of wrench faults inducing formation of continental basins with volcano-sedimentary sequences that are often coal bearing. The Variscan collision in central Europe was followed by periods of magmatic activity both within the orogen and on its foreland. Compositional differences between the volcanic rocks were controlled by magmatic sources reflecting different tectonic settings and mantle heterogeneity. Crustal thickening and southward-increasing depth of origin of mafic magmas in the North German Basin may reflect the presence of a pre-existing subduction-influenced basaltic magma source (Benek et al., 1996).

Subalkaline, transitional subalkaline–alkaline and more rarely alkaline volcanism is characteristic for the Variscan foreland basin (N Germany), and continental intermontane basins (Germany, Czech Republic, Poland) of the Variscan Orogen. During the Namurian–Westphalian, intermontane basins started to form parallel to the Variscan Orogen (Ziegler, 1990); Mattern (2001) designated these “Basin Family One”. The late Variscan intermontane basins of central Europe are aligned along pre-existing structural discontinuities and tectonic lineaments of the basement (strike directions E–W, NE–SW and/or NW–SE). The Variscan Orogen was subjected to the Late Paleozoic gravity collapse associated with basin formation parallel to the Variscan structural plan (Ménard and Molnar, 1988). The broad zone of the basins implies, together with intensive volcanism, a substantial tension and thinning of crust (Benek et al., 1996). The origin of the Permo-Carboniferous volcanic activity in the Bohemian Massif can be linked with the Variscan orogenic collapse. Initial Variscan (325–290 Ma) collisional shortening and uplift was followed at ca. 290 Ma by extension. This is documented by the widespread volcanic activity and thinning of the Permian crust (Scholle et al., 1995). However, Jindřich (1971) related the Permo-Carboniferous volcanism with the (half)-graben structures along Precambrian lineaments and/or strike-slip faults caused by the taphrogenic movements induced by updoming of the Bohemian Massif during the Late Paleozoic and continuing up to the Tertiary.

Nevertheless, the integrated studies on the Permo-Carboniferous volcanics of central Europe that consider both their structural position and geochemical (isotopic) signatures come from the last decade only (Seckendorff, 1990; Hoth et al., 1993; Korich, 1989, Korich, 1992; Benek, 1991, Benek, 1995; Benek et al., 1996; Kölbl-Ebert, 1995; Dziedzic, 1996a, Dziedzic, 1996b; Arz, 1996; Seckendorff et al., 2004; Awdankiewicz, 1999a, Awdankiewicz, 1999b; Schmidberger and Hegner, 1999).

Section snippets

Distribution of the Permo-Carboniferous volcanism in the Late Paleozoic basins of the Bohemian Massif

Products of intensive volcanic activity associated with the Variscan Orogeny are abundant in the sedimentary fill of the Late Paleozoic basins of the Bohemian Massif. Accumulations of largely synchronous ancient volcanic sequences are present in Central Bohemian basins, Lusatian or Sudetic basins: Česká Kamenice Basin, Mnichovo Hradiště Basin, Krkonoše Piedmont Basin, Intra-Sudetic Basin, see Fig. 1. For stratigraphic correlation of basic units in individual basins see Fig. 2. Notable coeval

Methods of investigation

More than 60 rock samples were collected for new petrographic and geochemical investigations of the Permo-Carboniferous volcanics in late Variscan continental basins of the Bohemian Massif. The Variscan basins are mostly covered by sediments of the Czech Cretaceous Basin and the majority of rock samples (about 80%) are from cores of deep boreholes. Samples from the Central Bohemian, Česká Kamenice and Mnichovo Hradiště basins originate exclusively from boreholes. Surface samples are mostly from

Permo-Carboniferous volcanism in the Late Paleozoic basins of the Bohemian Massif: a review

Late Variscan magmatism in the Bohemian Massif culminated during late Carboniferous to Permian. Magmas of (basalt)/basaltic (trachy)andesite to dacite–trachyte–rhyolite composition extruded as both lava flows and pyroclastic deposits. In some places subvolcanic intrusions are present. From the stratigraphic record two main episodes of volcanism can be discerned in the Central Bohemian Basins and Sudetic Basins:

  • 1.

    Volcanic products of the first episode (late Namurian to Stephanian B) form a

Central Bohemian Basins and West Bohemian Basins

Volcanics in the Central Bohemian Basins and West Bohemian Basins are restricted to the Carboniferous and they are small in number. Their degree of alteration is very strong often ruling out any attempt of chemical classification. They are confined to 37 horizons (Pešek, 1994) with rhyolite prevailing in the majority of tuffs and tuffites. Felsic compositions are more abundant among effusives sporadically occurring in the Bolsovian and in the Westphalian D to Cantabrian deposits of the Central

Considerations about possible source areas of the Permo-Carboniferous volcanics in the Bohemian Massif

Localizing the source areas and centers of volcanic activity is problematic wherever intrusive feeder facies are missing/not preserved (Fig. 6). The source areas of the first volcanic episode (Duckmantian to Stephanian B) are documented by effusive and shallow intrusive bodies in central and NW Bohemia. Pešek (1994) and Pešek (1998) consider that there were further source areas in W and NE Bohemia that have not been preserved. By contrast, Mašek (1973) considered the Saxonian part of the Krušné

Trace elements and REE patterns

A review of the trace element distribution is presented in multi-element variation diagrams and REE patterns of the Permo-Carboniferous volcanics from individual basins of the Bohemian Massif (see Fig. 5a and b). The most complete trace element information comes from the Krkonoše Piedmont Basin and the Intra-Sudetic Basin (cf. Ulrych et al., 2004, Ulrych et al., 2003).

In the Krkonoše Piedmont Basin volcanics, normalized multi-element variation diagrams of both episodes show very similar

Sr–Nd isotope systematics

As for the trace elements, the most complete information is from the Krkonoše Piedmont Basin and the Intra-Sudetic Basin.

In the Krkonoše Piedmont Basin all mafic samples have low initial εNd values between −2.7 and −6.0 and (87Sr/86Sr)i ratio (i  initial, t=290Ma) of ca. 0.706–0.707. The felsic rocks reveal a substantial range of εNd values (−0.9 to −5.1) and (87Sr/86Sr)i ratios (0.738–0.762). Epsilon values for rocks at their time of formation (t=290Ma) were calculated using the expression: εNd=

Origin of the felsic rocks

The Intra-Sudetic Basin rhyolitic rocks show affinities with the andesites, having similar trends in the normalized multi-element variation diagrams and REE patterns. Several specific characteristics, e.g., strongly negative Ba, Nb, Sr, P, Ti, Eu anomalies in the felsic rocks suggest plagioclase, apatite and titanian magnetite fractionation.

The notorious composition gap between intermediate and acid rocks in the Permo-Carboniferous volcanic series of the Bohemian Massif is partly filled by

Discussion

New geochemical studies of the Permo-Carboniferous volcanic rocks of the Bohemian Massif (Central Bohemian Basins  Jelínek et al., 2003; Sudetic Basins  Ulrych et al., 2004, Ulrych et al., 2003) show that the parental magma of both episodes (Carboniferous and Permian) was likely derived from the mantle. The geochemical characteristics of the volcanic rock series suggest that the primary magma was underplated at the mantle–crust boundary and then evolved in crustal chambers by AFC processes.

Conclusions

Extensive Permo-Carboniferous volcanism of the intermontane continental basins of the Bohemian Massif represents the late Variscan activity in central Europe. Volcanics of the first episode (Carboniferous Series  Ulrych et al., 2003) started at the Duckmantian–Bolsovian boundary and continued intermittently until the Westphalian D to the Middle Stephanian producing felsic–mafic volcanics. During the second episode (Permian Series  Ulrych et al., 2003), after the hiatus between Stephanian B and C

Acknowledgements

Financial support for this research was provided by the by Grant Project A301 3903 of the Grant Agency of the Academy of Sciences of the Czech Republic and the Scientific Programme CEZ: Z3-013-912 of the Institute of Geology, AS CR. The authors are grateful to E. Hegner, University München for isotope data and help with their interpretation. V. Lorenz, University Würzburg, E. Pivec, Institute of Geology, Acad. Sci CR, Prague, and V. Prouza, Czech Geological Survey, Prague, are thanked for

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